faulty earth connection,change plug.
http://www.western-towing.co.uk/acatalog/7pin_12S_Wiring.html
http://www.towitall.co.uk/faq/1.aspx
http://www.a-tconsulting.co.uk/caravan_tech/12s_probs.html
Use the 6 & 2 for the live & 7 &5 for the earth, i did mine 6 years ago no more problems.
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